This invention relates to methods and apparatus for avoiding undesirable growths, deposits and other formations in crystal growing operations; and, more particularly, to methods and apparatus for avoiding formation of projecting "whiskers" of silicon monoxide on the inner wall of a melt-containing crucible in Czochralski crystal growing operations.
It is conventional to provide single crystalline forms of many solid materials by preparing a melt of the material, and contacting the surface of the melt with a previously prepared seed crystal of the material of the desired crystalline lattice orientation. The seed crystal is withdrawn from the melt at a rate of the order of a few inches per hour, while the crystal and the melt are counter-rotated with respect to each other.
Typically, the chamber in which the crystal is grown is first partially evacuated and then backfilled to a positive pressure with a continuing flow of a gas, such as argon, which serves as the ambient during the crystal growth. The positive pressure aids in avoiding entrance of undesired contaminants into the system during the growth. With this described technique, commonly termed the Czochralski technique, crystals several feet in length and several inches in diameter are routinely grown, particularly by workers in the silicon semiconductor technologies.
Particularly, in the silicon semiconductor technologies, the melt, which may be at an average temperature of 1420.degree. C., typically is contained in a quartz (silicon dioxide) crucible. At the temperature involved, reaction of the quartz crucible with the molten material occurs, and thus provides a source of oxygen which, in turn, reacts with the molten silicon to produce silicon monoxide. The silicon monoxide is given off in vapor form from the melt and tends to preferentially condense, grow, or otherwise form dendrites, i.e., projecting growths, on what are apparently nucleation sites on the inner wall of the crucible just above the initial level of the melt surface during growth of the crystal.
These dendrites extend radially inward from the inner wall of the crucible, and often become dislodged and drop into the melt, where convection currents or other flow patterns can carry them to the growing crystal. Upon contact with the growing crystal, the desired crystalline structure usually is lost and any further growth would produce undesirable material.